Inductors are used in a wide range of applications, including, for example, communication systems, signal processing systems, filters, tank circuits, etc. As these electronic systems have become more integrated and scaled down, effectively, systems on a chip (SOC), circuit designers have sought to eliminate the use of large auxiliary components, such as inductors. When unable to eliminate inductors in their designs, engineers have sought to reduce the size of the inductors they do utilize so as to facilitate the inclusion of such inductors on-chip.
One approach to integrating inductors on-chip is to employ simulated inductors rather than discrete inductors. Simulating inductors using active circuits, which are easily miniaturized, is one approach to eliminating the use of actual inductors in electronic systems. Unfortunately, simulated inductor circuits frequently exhibit high parasitic effects, and often generate significantly more noise than circuits constructed using discrete inductors, and are therefore undesirable.
Inductors can be miniaturized for use in compact communication systems, such as cell phones, modems, etc., by fabricating spiral inductors on the same substrate as the integrated circuit (IC) to which they are coupled using IC manufacturing techniques. For example, most conventional implementations of inductance-capacitance (LC) tank oscillators utilize symmetrically designed integrated spiral inductors which are designed to optimize the performance of the individual inductors independent of any coupling properties. In some applications, such as, for example, SERDES (serializer/deserializer) and integrated radios, if there are two transmit and/or receive channels operating at slightly different frequencies (e.g., 400 parts-per-million (ppm) apart), then the inductors can couple magnetically and create unwanted interference signals. Moreover, spiral inductors take up a disproportionately large share of the available surface area on an IC substrate.
Conventional approaches for reducing inductor coupling have either involved lowering the current in the inductor, resulting in lower magnetic flux density, or physically spacing the inductors farther apart, since the coupling mechanism decreases as a function of the square of the distance between adjacent inductors. There have also been attempts at reducing coupling through the use of grounded shields (e.g., Faraday cage) around the spiral inductors. These methods, however, only reduce electric field interfere and not magnetic coupling, which is a primary source of coupling between spiral inductors.
Accordingly, there exists a need for techniques for forming IC inductors that do not suffer from one or more of the limitations exhibited by conventional approaches.